1. Field of the Invention
The present invention relates to a color conversion type organic electroluminescence device for extracting excited light of a luminescent material as external light by using luminescence radiated from an emitting layer as an excitation light source. Particularly it relates to an organic electroluminescence device by which luminescence as natural light radiated from an emitting layer can be extracted as polarized light rich in linearly polarized light and which is excellent in efficiency of extracting the luminescence. It also relates to a high-efficient (polarizing-type) planar light source and a high-efficient display device both using the organic electroluminescence device.
2. Description of the Related Art
An electroluminescence device or a light-emitting diode in which an emitting layer is provided between electrodes to obtain luminescence electrically has been researched and developed actively not only for application to a display device but also for application to various types of light sources such as a flat illuminator, a light source for optical fiber, a backlight unit for liquid crystal display, a backlight unit for liquid crystal projector, etc. Particularly, an organic electroluminescence device has attracted public attention in recent years because it is excellent in luminous efficiency, low-voltage drive, lightweight and low cost. A primary concern to the purpose of application to these light sources is enhancement in luminous efficiency. Improvement in cell structure, material, drive method, production method, etc. has been examined to obtain luminous efficiency equivalent to that of a fluorescent lamp.
In an inter-solid luminescent element such as an organic electroluminescence device in which luminescence is extracted from an emitting layer per se, however, light generated at an angle not lower than a critical angle decided on the basis of the refractive index of the emitting layer and the refractive index of an emergence medium is totally reflected and confined in the inside, so that the light is lost as guided light. According to calculation based on classical laws of refraction (Snell's law), light-extracting efficiency η in taking out generated light to the outside can be given by the approximate expression η=1/(2n2) in which n is the refractive index of the emitting layer. Assuming that the refractive index of the emitting layer is 1.7, then 80% or more of the light is lost as guided light, that is, as a loss in a side face direction of the cell because η is nearly equal to 17%.
In the organic electroluminescence device, excitons contributing to luminescence are only singlet excitons among excitons generated by recombination of electrons and holes injected from the electrodes. The probability that singlet excitons will be generated is ¼. That is, even in the case where only such a thing is considered, the efficiency is very low to be not higher than 5%.
As a technique for improving the luminous efficiency of the emitting layer per se, development of a luminescent material (JP-A-2001-313178) for generating light also from phosphorescence due to triplet excitons has advanced in recent years, so that the possibility that quantum efficiency can be improved remarkably has been found. Even if quantum efficiency were improved, luminous efficiency is reduced in accordance with light-extracting efficiency multiplied by the quantum efficiency. In other words, if light-extracting efficiency can be improved, there is room for remarkable improvement in luminous efficiency according to the synergy between the quantum efficiency and the light-extracting efficiency.
As described above, in order to take out guided light to the outside, a region for disturbing an angle of reflection/refraction need to be formed between the emitting layer and an emergence surface to destroy Snell's law to thereby change an angle of transmission of light originally totally reflected as guided light or beam-condensing characteristic need to be given to luminescence per se. It is however not easy to form such a region that outputs all guided light to the outside. Therefore, a proposal for taking out guided light as much as possible has been made.
For example, as techniques for improving light-extracting efficiency, there have been proposed a method in which beam-condensing characteristic is given to a substrate per se to improve light-extracting efficiency (JP-A-63-314795), a method in which an emitting layer is made of discotic liquid crystal to improve frontal directivity of generated light per se (JP-A-10-321371) and a method in which a stereostructure, an inclined surface, a diffraction grating, etc. are formed in the cell per se (JP-A-11-214162, JP-A-11-214163 and JP-A-11-283751).
These proposals, however, have a problem on complication in structure, reduction in luminous efficiency of the emitting layer per se, etc.
As a relatively simple technique, there has been also proposed a method in which a light-diffusing layer is formed to change an angle of refraction of light to thereby reduce light satisfying the condition of total reflection.
For example, there have been proposed various methods such as a method using a diffusing plate having a transparent substrate, and particles dispersed in the transparent substrate so as to form such a distributed index structure that the refractive index of the inside is different from the refractive index of the outside (JP-A-6-347617), a method using a diffusing member having a light-transmissive substrate, and a single particle layer arranged on the light-transmissive substrate (JP-A-2001-356207), and a method in which scattering particles are dispersed in the same material as that of the emitting layer (JP-A-6-151061).
These proposals have been provided by finding features in the characteristic of scattering particles, the refractive index-difference from a distribution matrix, the dispersing form of particles, the place for formation of the scattering layer, and so on.
Incidentally, the organic electroluminescence device uses such a principle that holes injected from the anode and electrons injected from the cathode by application of an electric field are recombined into excitons to generate luminescence from a fluorescent (or phosphorescent) substance. It is therefore necessary to perform the recombination efficiently in order to improve quantum efficiency. As this technique, there is generally used a method in which the cell is formed as a laminated structure. For example, a two-layer structure having a hole transport layer and an electron transport emitting layer or a three-layer structure having a hole transport layer, an emitting layer and an electron transport layer is used as the laminated structure. There have been also various proposals for a laminated cell formed as a double hetero structure in order to improve efficiency.
In such a laminated structure, recombination is substantially concentrated in a certain region.
For example, in the two-layer type organic electroluminescence device, as shown in FIG. 12, recombination is concentrated in an electron transport emitting layer side region 6 which is about 10 nm distant from an interfacial layer between a hole transport layer 4 and an electron transport emitting layer 5 which are sandwiched between a pair of electrodes constituted by a reflective electrode 3 and a transparent electrode 2 on a support substrate 1 (as reported by Takuya, Ogawa et al, “IEICE TRANS ELECTRON” Vol. E85-C, No. 6, p. 1239, 2002).
Light generated in the emitting region 6 is radiated in all directions. Consequently, as shown in FIG. 13, an optical path difference is produced between light radiated toward a light-extracting surface on the transparent electrode 2 side and light radiated toward the reflective electrode 3, reflected by the reflective electrode 3 and radiated toward the light-extracting surface.
In FIG. 13, the thickness of the electron transport emitting layer in the organic electroluminescence device is generally in a range of from tens of nm to a hundred and tens of nm, that is, the order of wavelength of visible light. Accordingly, light beams finally coming out of the cell interfere with each other. The interference becomes destructive or constructive according to the distance d between the emitting region and the reflective electrode.
Although only light radiated in a frontal direction is shown in FIG. 13, light radiated in oblique directions is also present actually. The condition of interference varies according to the angle of radiated light in addition to the distance d and the wavelength λ of generated light. As a result, there may occur the case where light beams radiated in a frontal direction interfere with each other constructively but light beams radiated in a wide-angle direction interfere with each other destructively, or there may occur the case reverse to the aforementioned case. That is, luminance of generated light varies according to the viewing angle. It is a matter of course that the intensity of light varies remarkably according to the angle as the distance d increases. Therefore, the thickness of the electron transport emitting layer is generally selected so that the distance d is made equal to about a quarter of the wavelength of generated light to obtain constructive interference of light in the frontal direction.
When, for example, the distance d is smaller than about 50 nm, absorption of light becomes remarkable in the reflective electrode generally made of a metal. This causes reduction in intensity of generated light and influence on intensity distribution. That is, in the organic electroluminescence device, the distribution of radiated light varies remarkably according to the distance d between the emitting region and the reflective electrode, so that the guided light component varies widely according to the variation in the distribution of radiated light.
Furthermore, the emission spectrum of the organic electroluminescence device has broad characteristic in a relatively wide wavelength range. Accordingly, variation in the wavelength range for constructive interference of light according to the distance d causes variation in peak wavelength of generated light. Furthermore, the emission spectrum varies according to the viewing angle in addition to the distance d.
To solve these problems, there has been made a proposal for selecting the film thickness to suppress a phenomenon that the color of generated light varies according to the viewing angle (see Patent Document 1). In this proposal, however, there is no description concerning guided light. It is obvious that the film thickness range selected by this proposal for suppressing the dependence of the color of generated light on the viewing angle is different from the range according to the invention which will be described later.
For the aforementioned reason, the light-extracting efficiency of the laminated organic electroluminescence device cannot be calculated correctly on the classical assumption that about 80% of generated light is confined as guided light in the inside of the cell. That is, the guided light component varies remarkably according to the structure of the cell. For example, as reported by M. H. Lu et al (J. Appl. Phys., Vol. 91, No. 2, p. 595, 2002), detailed research on change in the guided light component according to the structure of the cell has been made on the basis of a quantum-mechanical calculation method in consideration of a micro-cavity effect.
Accordingly, there is a possibility that the obtained effect will not be so large as estimated by the classical theory even in the case where a light-diffusing layer or the like is formed in order to destroy the condition of total reflection.
On the other hand, there has been proposed a color conversion technique in which a luminescent material for generating fluorescence in a visible light range when absorbing luminescence generated from an organic electroluminescence device is used as a filter (see Patent Documents 2 and 3). In this technique, arbitrary visible light such as white light can be extracted as generated light when the kind of luminescent material, the amount of addition of luminescence material, the mixture ratio of materials, etc. are adjusted. Furthermore, when full color display needs to be achieved in a display device, it is generally necessary to form red, green and blue organic luminescence layers in accordance with each pixel. In the aforementioned technique, full color display can be achieved when all pixels are formed as an organic electroluminescence layer while a color filter separately painted with red, green and blue luminescent materials is used separately. As a method for producing the color filter, a conventional technique cultivated for producing a color filter used for a liquid crystal display device can be applied as it is. Reduction in production cost can be also made.
When the luminescent material absorbs excitation light from the organic electroluminescence device and generates light, the situation that large part of the generated light is confined as guided light in the inside of the cell is however unchanged because the light is generated in a solid, in addition to lowering of efficiency due to light conversion efficiency. On the contrary, when a conventional technique in which particles different in refractive index from the matrix are dispersed in the region of the dispersed luminescent material or a conventional technique in which a light-diffusing layer or a lens sheet is formed on the luminescent material is used, guided light can be extracted to the outside to a certain degree.
When the organic electroluminescence device is used as a backlight unit for a liquid crystal display device, luminescence radiated from the cell need to be converted into linearly polarized light by a polarizing plate for the purpose of liquid crystal display because the luminescence is natural light. As a result, absorption loss due to the polarizing plate is produced. There is a problem that the rate of utilization of light cannot be set to be higher than 50%.
As a technique for solving this problem, there has been made a proposal for forming an organic electroluminescence device layer on an oriented film to take out luminescence per se as linearly polarized light (see Patent Document 4). Although the absorption loss due to the polarizing plate can be reduced to half, at the most, by this proposal, there is a possibility that the luminous efficiency of the cell will be lowered because of the insertion of the oriented film etc. for orienting an organic thin film. In addition, like the background-art cell, the problem of guided light due to total reflection cannot be solved at all by this proposal. Even if a light-diffusing layer were formed in such a linearly polarized light-emitting cell, the linearly polarized light will be scattered and converted into natural light nonsensically.
The applicant of the present invention has already proposed a method in which light generated in an organic electroluminescence device is extracted through a polarizing/scattering film (see Patent Document 5). According to this proposal, light lost as guided light can be scattered so as to be extracted, and output light can be extracted as polarized light which is rich in linearly polarized light. Accordingly, the absorption loss due to the polarizing plate can be reduced, so that a polarizing-type planar light source of high efficiency can be provided as a light source for a liquid crystal display device.
For example, the relation between the guided light and the influence of the distance between the emitting region and the reflective electrode on interference has not been described yet in this proposal. It cannot be said that this proposal brings out the greatest possible effect of the light source for a liquid crystal display device.
As described above, no report in the background art gives attention to detailed examination of improvement in light-extraction efficiency on the assumption that detailed research is made on change in guided light component in accordance with the cell structure while the characteristic of the color conversion technique that arbitrary visible light can be obtained as polarized light is used. Therefore, the provision of a polarizing-type planar light source of high efficiency best adapted to a liquid crystal display device using polarized light is desired earnestly in the existing circumstances.
[Patent Document 1]
JP-A-5-3081
[Patent Document 2]
JP-A-3-152897
[Patent Document 3]
JP-A-5-258860
[Patent Document 4]
JP-A-11-316376
[Patent Document 5]
JP-A-2001-203074